Natural gas found in deposits in the earth is an abundant energy resource. For example, natural gas commonly serves as a fuel for heating, cooking, and power generation, among other things. The process of obtaining natural gas from an earth formation typically includes drilling a well into the formation. Wells that provide natural gas are often remote from locations with a demand for the consumption of the natural gas.
Thus, natural gas is usually transported large distances from the wellhead to commercial destinations in pipelines. This transportation presents technological challenges due in part to the large volume occupied by the gas. Because the volume of a gas is so much greater than the volume of a liquid containing the same number of molecules, the process of transporting natural gas typically includes chilling and/or pressurizing the natural gas in order to liquefy it. Unfortunately, this liquefaction contributes to the final cost of the natural gas.
Further, naturally occurring sources of crude oil used for liquid fuels, such as gasoline and middle distillates, have been decreasing, and supplies are not expected to meet demand in the coming years. Middle distillates typically include heating oil, jet fuel, diesel fuel, and kerosene. Because those fuels are liquid under standard atmospheric conditions, they have the advantage that in addition to their value, they do not require the energy, equipment, and expense of the liquefaction process. Thus, they can be transported more easily in a pipeline than natural gas.
Therefore, for all of the above-described reasons, there has been an interest in developing technologies for converting natural gas to more readily transportable liquid fuels, i.e. to fuels that are liquid at standard temperatures and pressures. One method for converting natural gas to liquid fuels involves two sequential chemical transformations. In the first transformation, natural gas or methane (the major chemical component of natural gas) is reacted with oxygen to form synthesis gas (syngas), which is a combination of carbon monoxide gas and hydrogen gas. In the second transformation, known as the Fischer-Tropsch process, carbon monoxide is reacted with hydrogen to form organic molecules known as hydrocarbons, which contain carbon and hydrogen atoms. Other organic molecules known as oxygenates, which contain oxygen in addition to carbon and hydrogen, also may be formed during the Fischer-Tropsch process.
The Fischer-Tropsch product stream commonly contains a range of hydrocarbons, including gases, liquids, and waxes. It is desirable to primarily obtain hydrocarbons that are liquids and waxes, e.g., C5+ hydrocarbons that may be processed to produce fuels. For example, the hydrocarbon liquids may be processed to yield gasoline, as well as heavier middle distillates. The hydrocarbon waxes may be subjected to additional processing steps for conversion to liquid hydrocarbons.
The Fischer-Tropsch process is commonly facilitated by a catalyst having the function of increasing the rate of reaction without being consumed by the reaction. A feed containing syngas is contacted with the catalyst in a reaction zone that may include one or more reactors. Common catalysts for use in the Fischer-Tropsch process contain at least one catalytic metal from Groups 8, 9, or 10 of the Periodic Table (based on the new IUPAC notation, which is used throughout the present specification). Cobalt metal is a particularly desirable catalytic metal in catalysts that are used to convert natural gas to heavy hydrocarbons suitable for the production of diesel fuel. Alternatively, iron, nickel, and ruthenium have served as the catalytic metal. Nickel catalysts favor termination and are useful for aiding the selective production of methane from syngas. Iron has the advantage of being readily available and relatively inexpensive but the disadvantage of a high water-gas shift activity. Ruthenium has the advantage of high activity but is quite expensive.
The catalysts often further employ a promoter in conjunction with the principal catalytic metal. A promoter typically improves one or more measures of the performance of a catalyst, such as activity, stability, selectivity, reducibility, or regenerability. In addition to the catalytic metal, a Fischer-Tropsch catalyst often includes a support. The support is typically a porous material that provides mechanical support and a high surface area upon which the catalytic metal and any promoter are deposited.
The method of preparation of a catalyst may influence the performance of the catalyst in the Fischer-Tropsch reaction. In a common method of loading the catalytic metal to a support, the support is impregnated with a solution containing a dissolved metal-containing compound. When a promoter is used, an impregnation solution may further contain a promoter-containing compound. After drying the support, the resulting catalyst precursor is calcined, typically by heating in an oxidizing atmosphere, to decompose the metal-containing compound to a metal oxide. The preparation of the catalyst may include more than one impregnation, drying, and calcination cycle. When the catalytic metal is cobalt, the catalyst precursor is then typically reduced in hydrogen to convert the oxide compound to reduced “metallic” metal. When the catalyst includes a promoter, the reduction conditions may cause reduction of the promoter, or the promoter may remain as an oxide compound. As a result of the method described above, the catalyst precursor becomes an activated catalyst capable of facilitating the conversion of syngas to hydrocarbons having varying numbers of carbon atoms and thus having a range of molecular weights.
Catalyst supports employed for the Fischer-Tropsch process have typically been refractory oxides (e.g., silica, alumina, titania, thoria, zirconia or mixtures thereof, such as silica-alumina). It has been asserted that the Fischer-Tropsch reaction is only weakly dependent on the chemical identity of the metal oxide support (see E. Iglesia et al. 1993, In: “Computer-Aided Design of Catalysts,” ed. E. R. Becker et al., p. 215, New York, Marcel Dekker, Inc.). Nevertheless, because it continues to be desirable to improve the activity of Fischer-Tropsch catalysts, other types of catalyst supports are being investigated. The physical characteristics of a supported catalyst tend to influence the performance of the catalyst. In particular, the dispersion of the catalytic metal may influence the performance of a supported catalyst. Lower dispersion does not fully utilize metal sites and is not an efficient use of the available metal. In contrast, high dispersion of catalytic metal more fully utilizes metal sites and results in a higher initial catalyst activity, such as carbon monoxide conversion in the Fischer-Tropsch reaction. However, highly dispersed metal tends to be more difficult to reduce and tends to deactivate more rapidly.
Support acidity is another factor contributing to the efficacy of catalyst supports and the catalysts made therefrom. The acidity of a catalyst support can manifest itself as Bronsted or Lewis acidity by the presence of bonded protons or electron deficient centers, respectively. Such acidity can have a two-fold effect upon catalysts made therefrom. The acidity of a support upon which a catalytic metal is dispersed may influence the nature of the resulting dispersion of metal. Secondly, an acidic catalyst support in the presence of a dispersed catalytic metal results in a bifunctional catalyst. Such bifunctional catalysts usually find utility in various hydroprocessing catalysts, but it seems according to some in the art that varying surface acidity present in some combinations of support and metal, i.e., ruthenium on titania, does affect catalyst activity in hydrocarbon synthesis processes. One method to measure support acidity has been proposed by R. L. Espinoza et al. in “Catalytic Oligimerization of Ethene over Nickel-Exchanged Amorphous Silica-Alumina: Effect of the Acid Strength of the Support,” Applied Catalysis 29, pp. 295-303 (1987), hereby incorporated by reference herein to the extent that it discloses such method. In Espinoza et al. (1987), the acidity index is proposed to quantify the acidity found on the exposed surface of a catalyst support. The acidity index preferably is determined according to the procedure that R. L. Espinoza et al. described.
Thus, it is desirable to control the dispersion of metal for supported metal catalysts. Methods for controlling dispersion typically involve a modification of the method of depositing metal on the support. For example, methods of affecting the dispersion of a catalytic metal include controlling the evaporation rate of the solvent (e.g. water) used to dissolve the metal precursor (e.g. nitrates or acetates of Fe, Co, Ru, and the like) whether achieved by varying the temperature or pressure. Other techniques for adjusting dispersion include using solvents with higher or lower boiling points than water. Further, chemical vapor deposition is a well-known technique for controlling the final dispersion of metal.
Alternatively, a method for controlling the dispersion may include selecting the catalyst support material. Although catalyst supports are desirably inert, it is known that the identity of a support may influence the dispersion of metal deposited on the support. For example, metal deposited on alumina tends to be characterized by higher dispersion than metal deposited on silica.
Varying the average pore diameter of the support is another technique to control dispersion. In general, a correlation is present between the average pore size of the support and the average metal or oxide crystallite size. Due to steric constraints, the metal or oxide crystallite size cannot be larger than the pore, although it may be smaller. This technique has the disadvantage that, in order to control the average metal or oxide crystallite size, the average pore size has to be manipulated. This may result in negative effects such as diffusional constraints, low support surface areas or lower support mechanical strength, among others.
Thus, there remains a need for supported catalysts having controlled dispersion of catalytic metals or oxides and methods of making such catalysts.